Digital color-signal-processing circuits are adapted to the color-coding technique used in the NTSC, SECAM, PAL and related color-television standards, such as D2-MAC. This is the usual coding of the chrominance signal by means of the R-Y and B-Y color-difference signals, which precisely specify the location of the respective picture element in the color plane. See, for comparison, the CIE chromaticity diagram according to the German standard DIN 5033. Since the respective luminance value ("Y") is present as the Y signal, saturation, and luminance are unambiguously defined for the respective picture element by these three signals.
As is well known, this coding technique makes it possible to simultaneously transmit the chromaticity information, namely the respective hue and the respective saturation, within the spectrum of the Y signal, thus permitting monochrome reception of the transmitted color picture signal.
In the NTSC and PAL standards, the two color-difference signals are transmitted simultaneously as quadrature-modulated waves using a single chrominance subcarrier whose phase and amplitude are modulated and whose frequency lies in the range of the Y signal.
In the SECAM standard, one of the two color-difference signals is transmitted on one line as a frequency-modulated wave using a first chrominance subcarrier, and the other color-difference signal is transmitted on the next line as a frequency-modulated wave using a second chrominance subcarrier. As in the PAL/NTSC standard, the modulation range of the two chrominance subcarriers lies within the frequency range of the Y signal.
In the D2-MAC standard, the two color-difference signals are transmitted on alternate lines as in the SECAM standard, but at baseband, because, instead of transmitting the chromaticity information within the Y-frequency range using frequency-division multiplexing, a time-division multiplexing method is used in which the chromaticity information and the Y signal are separated in time within a line.
After the R-Y and B-Y color-difference signals have been separated from the composite color signal, e.g., by demodulating the chrominance subcarrier, color-signal processing is performed with the two R-Y and B-Y signals as usual. In digital color-signal-processing systems, these two signals and the Y signal are present as digital data, i.e., digitization takes place before or after the separation; however, this does not form part of the subject matter of the present invention.
The Y signal is fed to a separate Y-processing stage and subsequently combined with the two color-difference signals in a color matrix to generate the values for the red (R), green (G), and blue (B) signals. The dematrixing does not form part of the present invention, either.
Because of the combination of hue and saturation in the two R-Y and B-Y color-difference signals, the processing of these signals affects both components. On the other hand, both signals must be processed, even if only the hue or only the saturation is to be changed.